The present invention relates to amplifiers and, more particularly, to high frequency power amplifiers utilized in systems requiring operation with variable load impedances.
In many present communications systems, especially high frequency systems, the output frequency is changed at fast or slow rates in order to provide frequency hopping for making the system more secure. In such systems, a power amplifier typically receives a modulated carrier frequency, which carrier frequency is then changed in accordance with the frequency hopping scheme. The output of the power amplifier is then coupled through an antenna matching device or directly to an antenna for transmission of the modulated carrier.
As will be appreciated, in order to provide the most efficient operation, there must be an impedance match between the power amplifier and the antenna or coupling network. When considering such things as reliability and noise generation in connection with the need to provide that impedance match, it is not desirable to mechanically or electrically alter the network, such as by use of a coupler, between the power amplifier and an antenna. Thus, the system should be constructed so that there is no such need for the change in the coupling network while still achieving broadband operation. However, use of a broadband antenna or antenna coupler combination at high frequencies cannot be expected to provide an impedance match better than 50 ohms within a VSWR of 2:1 or 2.5:1.
In any such systems where the power amplifier is designed to produce power output into a nominal 50 ohm load, but operates with VSWRs up to 2.1 or 2.5:1, there will be a decrease in efficiency. Specifically, at high load impedances, such an amplifier will tend to distort the amplified signal, while at low load impedances, the amplifier will tend to dissipate large amounts of power in the output of the amplifier, both of which are undesirable in system operation. In each instance, the problem arises because the supply voltage of the power amplifier (Vcc or Vdd) driving the power transistors is maintained constant in spite of the variations in the load impedance, thereby resulting in the decrease in efficiency or distortion of the amplified signal depending upon the variation in the load impedance. There is therefore a continuing need for amplifiers which are capable of operating in broadband applications having wide variations in load impedance.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide an improved power amplifier system and technique for broadband, high frequency applications.